Fix code sample indentation in arbre documentation

[amiel]

The tab-style indentation in docs/12-arbre-components.md makes for not-so-pretty code samples.

This pull-request changes tabs in to 2 spaces. Except for the first tab, which needs to be 4 spaces for code samples.

[seanlinsley]

What. Just what. I don't know how to respond to this.

This is on the main activeadmin.info website, right now.

[seanlinsley]

@gregbell, this happened when you merged #1745

[amiel]

Yeah, I just noticed the new documentation. Which I really appreciate. I've learned a lot of ActiveAdmin tricks by reading through the internals and other examples, but this documentation is nice to have.

I also just realized that although I fixed the indentation of code samples, I didn't do anything to the headers, which also look like they might need some adjusting...

[seanlinsley]

Yes, that file is absolutely terrible. If you could fix it up some more, I'd really appreciate it :)

[amiel]

@daxter Actually, the hierarchy is pretty sane. I'm not really sure what to do with the headings...

[amiel]

Oh, nevermind, I see: https://github.com/carnesmedia/active_admin/blob/9d82ed17a8dadcd47e4dc834e17dc1ecce185a93/docs/12-arbre-components.md#index_as_table-table_for-takes-a-collection-and-a-hash-of-options-and-then

So, how do I render the documentation to test my changes?

[amiel]

@daxter That should do it ^^^^^

[amiel]

ug, I keep on forgetting to [ci skip]

[seanlinsley]

Right now the heading hierarchy is:

# Arbre Components
## Text Node
## Common Components
### Panel Component
### Columns Component
#### Simple Columns
#### Multiple Span Columns
#### Max and Mix Column Sizes
### Table For Component
### Status tag

I would:

• remove every use of the word "component" except the very first one
• remove the "Common Components" header entirely, and move every sub-heading up one level in importance (so ### becomes ##)
• rename "Max and Mix Column Sizes" to "Custom Column Widths"

[amiel]

@daxter Updated per your suggestions

[seanlinsley]

Cool. This makes a huge difference to readability. Thank ya!